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Chapter 71: Pulses
A pulse is a single event in a medium.
An example of a pulse is shown.
An upwardly pointing pulse is directed from the left to the right if a string is fixed at both ends and made taut.
The pulse appears to travel down the string.
The energy transferred to the string causes segments to vibrate up and down.
The illusion of a continuous pulse is created by this effect.
The pulse is said to be transverse because it is only energy that is transferred by it.
The level of the string affects the pulse's displacement above it.
The velocity of the pulse increases if the tension in the string is increased.
The pulse appears to move slower if a heavier string is used.
The velocity of waves is dependent on the medium.
Waves and pulse travel through the medium.
When the pulse reaches one boundary, the energy is directed against the wall and creates an upward force.
The reaction to this action is a downward force because the wall is rigid.
The string is reflected back after being displaced downward.
The result of the boundary interaction is seen as an inverted pulse upon reflection.
Most of the energy is reflected back by the pulse.
Waves transfer energy.
After the vibrations pass through, the medium returns to its original state.
A different effect occurs when there are two strings of different mass.
The second string is displaced upward when the pulse reaches the nonrigid boundary.
The mass density and tension in the second string affect the magnitude of the second displacement.
The pulse may travel with a larger or smaller velocity depending on the mass density.
If the difference between the two media is large enough to act as a semirigid boundary, the reflected pulse will be upward in orientation.
A light string attached to a heavy rope has a pulse travel from the string to the rope.
When two pieces of matter interact, they obey the same set of physical laws as if they were two pieces of energy.
The principle of superposition states that when two pulse interact at the same point and at the same time, the interaction produces a single pulse with the same displacement as the original two pulse.
Both pulse continue in their original directions after the interaction.
This interaction can be called interference.
When a pulse with an upward displacement interacts with another pulse with an upward displacement, the resulting pulse has a larger displacement than the original pulse.
The interaction is called constructive interference.
When a pulse with an upward displacement interacts with a pulse that has a downward displacement, the resulting pulse has a smaller displacement than the original pulse.
In the destructive interference case, it is possible that the interaction will temporarily cancel out the two pulse.
The orientation of the pulse depends on the magnitude of the pulse.
A wave train is a set of waves created by a continuous up and down vibrating string.
A wave consists of a complete up and down segment.
Waves are part of a medium.
The letter f designates the number of waves per second and is expressed in seconds orhertz.
The period is the amount of time required to complete one wave cycle.
The wave's period and Frequency are related to each other.
Waves do not transmit any energy.
The medium vibrates up and down as the waves approach.
Many points in the medium will be moving up or down at the same time.
The phase of a wave is the relative position of a point on a wave with respect to another point on the same wave.
The Greek letter l (lambda) designates the distance between two points in phase as a measure of the wavelength.
The peaks of the wave are called crests.
One wavelength can be used to measure the distance between crests and troughs.
The wave's distance, measured in meters, is the maximum displacement, up or down, above the normal rest position.
The wavelength, Frequency, and speed of the wave are related.
The wave is similar to a graph of displacement versus time for a mass on a spring.
The maximum energy in a simple motion system is proportional to the square of the amplitude.
This is the case with a simple wave motion.
We can identify points A and A in this illustration.
The distance from B to B can be used to measure the wavelength.
The Frequency is proportional to the energy of a wave.
The wave's intensity is related to the amplitude.
The property might be called volume in sound.
The wave's pitch is measured by the Frequency.
There are many different frequencies that interact with humans.
The kilo, mega, and giga represent 10 3, 10 6 and 10 9 respectively.
There are two types of waves in physics.
These waves do not need a medium to carry them.
All waves are in nature.
Pulling the spring back and forth along its longitudinal axis is one way to set up periodic disturbances in the spring.
There are regions of compressions and expansions that travel parallel to the disturbances themselves.
The waves are called longitudinal or compressional.
A longitudinal wave is an example of sound.
The phenomenon known as sound is created by the pressure differences in the air that are created by the sound of an object in the air.
You need to know the differences between the waves.
You should be aware of the differences between mechanical and electromagnetic waves.
There are a number of ways in which we can make the string vibrate.
The entire string can be moved up and down as a single unit.
The fundamental mode is the easiest mode of vibration to do.
Something interesting can happen if we increase the frequencies.
The waves reaching a boundary reflect off it inverted and match perfectly.
In this situation, the wave appears to stop in the horizontal motion and we have a standing wave.
There are points where no displacement takes place.
One nodal point appears in the second mode.
The nodal point occurs at the one-half wavelength mark.
In the fundamental mode, l is 2l.
There are two nodal points and l is 2l/3.
The nodal points would be determined by a drop in intensity of the standing-wave pattern.
We would see a darker region with light.
We can see the pattern with a string.
Each segment is half a wavelength and has an antinode in the middle.
They are destructive interference.
Constructive interference is caused by antinodes.
Waves can be manipulated to reinforce themselves.
There is a build up of wave energy due to the interference of standing waves.
An external agent can cause resonance.
All objects vibrate.
When a glass or bell is struck, the sound is a complex mixture of vibrations.
The phenomenon of resonance can be observed if the tuning forks A and B are held close together.
Waves from fork A strike fork B if tuning fork A is struck.
Since the waves match the natural vibrating frequencies of fork B, that fork will begin to vibrate.
An opera singer can break a crystal wine glass.
It explains why soldiers are ordered to "break march" when crossing a bridge.
The bridge can be Collapsed if a resonance vibration is set up by the rhythmic marching.
The Tacoma Narrows Bridge in Washington State was damaged in November 1940 by a resonance vibration caused by a light gale wind.
The images of the convulsions are a classic example of the principle of resonance.
The bridge has become a testament to the need for engineers to be very careful when considering the effects of resonance.
The sound is a mechanical wave.
As pressure differences move through the air, air molecule spacing is rare.
The sound of thunder is created when air expands due to the high temperature of a lightning bolt.
It is temperature dependent since sound is carried by air.
The sound in the air is 331 meters per second.
The study of sound and longitudinal waves in matter are called acoustics and acoustical waves, respectively.
The structure of the matter affects the ability of sound waves to pass through it.
Sound travels slower in a gas, in which the molecule have more random motion, than in a liquid, which has a more rigid structure.
The velocities of sound in substances are listed.
Sound waves can also be formed by standing waves.
All musical instruments work by creating waves in the instrument that correspond to the frequencies of the notes being created.
The standing sound wave is an interference of the wave and its own reflection from either a closed or open end of a tube.
It depends on whether they are modeling the standing sound wave as a pressure wave or a displacement of the molecule wave.
The closed ends are used for pressure and displacement.
Open ends can be used for displacement or pressure.
Imagine a student blowing over the top of an empty bottle and hearing a specific pitch for the standing air wave produced by the column of air in the bottle.
There is a variation of this problem where the column of air is adjusted.
Half of the first wave in a standing wave will look like one at one end and another at the other.
The speed of sound in the air is approximately 340 m/s.
Musical instruments are constructed to be quarter-wavelength pipes that are closed on one end, just like this example.
The medium is used to set the wave speed.
The source sets the wave frequency.
Human hearing can detect sound up to 20,000 hertz.
Ultrasonic waves are sound waves over 20,000 hertz.
The intensity of the sound wave is related to the pitch of the wave.
The regions of constructive and destructive interference produce beats when two sound waves interfere.
The number of beats per second is the same as the Frequency difference.
The path-length difference is what determines interference for any wave.
Two point sources of sound, A and B, and a receiver are some distance away.
If the path-length difference is equal to a whole multiple of the wavelength, the receiver will be at a point of constructive interference.
Diffraction is a phenomenon associated with interference.
The wave appears to bend around the corners of the boundary when it encounters a boundary.
Diffraction occurs because at the corners the wave behaves like a point source and creates circular waves.
Because of their shape, these waves reach behind the corners and give the illusion of wave bending.
A series of straight waves, made in a water tank, are observed in the film.
The wavelength of wave present is relative to "Narrow".
Waves don't show a lot of diffraction through openings larger than their wavelength.
Everyone has heard a siren.
The change in position of the siren relative to the hearer causes a change in the sound.
As the siren approaches, the pitch is increased while the pitch is decreased.
The phenomenon is called the Doppler effect.
Let's say the source is moving toward point A.
The circular waves will not be in a straight line for each period of time.
The spacing between the two waves will be reduced by an amount equal to the distance traveled by the source.
At point A there is an apparent increase in Frequency, while at point B there is an apparent decrease in Frequency.
The relative motion between the source of the wave and the receiver is what causes this shift.
A combination of source and observer movement can be used to accomplish this.
The received Frequency will be higher if the two are near each other.
The received Frequency will be lower when the two are further apart.
The relative velocities of the two are related to the amount of the shift in frequencies.
There are mechanical waves in a medium.
A pulse is a single event.
The direction of propagation is what determines whether a pulse or wave is transverse.
If the waves are parallel to the direction of motion, they are longitudinal.
Waves are only able to transmit energy.
Waves can be added together with the principle of superposition states.
The maximum displacement of a wave is called the amplitude.
The number of cycles per second is the Frequency of a Wave.
The period of a wave is the amount of time it takes to complete one cycle.
Particles in a medium that show the same state of motion at the same time are said to be in phase.
The wavelength is the distance between two points in a wave.
Waves are equal to the product of the frequencies.
A standing wave is a wave in which incident and reflected waves combine to produce a wave that appears to be standing in one place.
There are points where there is no displacement.
Every half-wavelength, there are Nodes.
The maximum energy transfer from one body to another is known as resonance.
A difference in media at a boundary causes the wave to return in the opposite direction.
The bending of a wave as it enters a new medium at an oblique angle has different propagation speed.
Diffraction is the bending of waves around obstacles in a given medium or the outward speed as the wave passes through small openings.
Constructive or destructive interference can be produced by the superposition of two waves of the same phase.
Sound is a mechanical wave.
There is an apparent increase in frequencies when a source is moving toward an observer.
There is an apparent decrease in frequencies if the relative motion is away from the source.
Remembering the basic physical concepts is needed to solve wave motion problems.
The only motion in the medium will be up and down.
If you see points labeled as beads on a wire, you must resist the temptation.
Waves will first go up and then go down as they approach a point.
The points are either going up or going down at the same time.
Sound waves travel through a medium and their speed is determined by the structure of the matter involved.
The speed of sound increases with the temperature.
When two frequencies are heard at the same time, sound wave interference occurs.
A stretched string has a length of 1.5 m and a mass of 0.25 kg.
A stretched string is vibrated in such a way that a wave appears.
The tuning forks are vibrating.
There are two forks with different frequencies, one with a Frequency of 256 and the other with a Frequency of 280.
A sound wave can take 3 s to reach a receiver.
When neighbors play their stereos too loud, apartment dwellers are used to hearing the "boom-boom" sound of bass tones.
T is 4.2 N if you use the given information and solve for tension.
The wavelength of l is 1.14 m.
Waves are perceived as being louder in sound.
T is 48,000 N.
When crossing a bridge, people should not march in unison because it can lead to destructive large amplitudes.
The bass tones in the apartment building walls are so low that they sound like "boom-boom", which is familiar to apartment building dwellers.
The sound waves are carried farther by the hot summer air.
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